1. Field of the Invention
The invention relates to an automotive heating, venting and air conditioning (HVAC) system and, more specifically, to a vapor compression air conditioning system augmented by an evaporative cooler operating in series with the air conditioning system.
2. Description of the Prior Art
The HVAC system to which the subject invention pertains includes a housing with an evaporative cooler supported in the housing upstream of an evaporator core for receiving primary air from dry channels of the evaporative cooler, which also defines a plurality of wet channels extending transversely to the dry channels for receiving secondary air. A wicking tank is disposed under the wet channels for containing liquid and a wicking material is disposed in the wet channels for drawing liquid from the wicking tank into the wet channels. A reservoir is supported by the housing for collecting liquid condensate from the evaporator core.
A prime function of the automotive air conditioning system is to supply properly conditioned air to the passenger compartment of the vehicle. The conditioned air is obtained by reducing the dry bulb temperature of the moisture-laden hot air admitted into the air conditioning system. When such an air stream flows through the air conditioning system, there is transfer of both sensed or sensible heat (sensed by a thermometer) and latent heat (hidden and not sensed by the thermometer). The air conditioning system thus conditions the air by cooling it in response to the sensed heat and additionally by removing moisture therefrom. The removal of moisture from air occurs exothermally which means that the moisture is condensed from the moist air as its dry bulb temperature increases. The temperature of the air rises about 0.75° F. for each grain of moisture (1 grain=0.000143 lbm) condensed therefrom. Thus, the air conditioning load has two distinct components: sensed or sensible load due to cooling alone (i.e., drop in dry bulb temperature) and latent load due to moisture removal from air with concurrent rise in its dry bulb temperature. The evaporative cooling system is best suited for sensible load reduction.
The evaporative cooling system employs two separate airstreams—primary and secondary. The primary air flows through the dry channels of the evaporative cooler while the secondary air flows through the wet channels of the evaporative cooler. The wet channels are lined with a wicking material, which holds liquid water for evaporation. The heat required for evaporation is abstracted from the primary air stream flowing in the contiguous dry channels. Thus, the liquid water evaporating in the wet channels lowers the dry bulb temperature of the primary air in the dry channels.
The two principal methods of evaporative cooling are direct evaporative cooling and indirect evaporative cooling. A variant of the indirect evaporative cooling method, called staged indirect evaporative cooling, has also found applications in recent years as described in the U.S. Pat. No. 5,453,223 to Maisotsenko; U.S. Pat. Nos. 6,497,107; 6,581,402 and 6,705,096 to Maisotsenko et al.
In the direct evaporative cooling method, there are no dry channels so that the primary air flows through the wet channels. During its passage through the wet channels, the dry bulb temperature of the primary air decreases while its absolute humidity increases due to vaporizing liquid water in the wet channels. However, a decrease in the dry bulb temperature is desired to result in lower sensible air conditioning load, as an increase in the absolute humidity is undesired to result in higher air conditioning load. Thus, direct evaporative cooling is counterproductive to some extent.
In the indirect evaporative cooling method, the primary air flows through the dry channels and the secondary air through the wet channels. The two air streams do not come in direct contact with each other so as to keep the absolute humidity of the primary air at its initial level. However, the absolute humidity of the secondary air increases as it flows through the wet channel due to the vaporization of the liquid water on the wet channel walls, as carried by the wicking material. As a result, the temperature of the wet channel wall is lowered. The primary air flowing through the dry channels tends to assume the temperature of the cooled wet channel walls without absorbing any moisture and thereby maintaining its absolute humidity at the original level. It becomes apparent that, in indirect evaporative cooling, the primary air is cooled sensibly with a heat exchange through the walls of the dry channels as secondary air flowing through the wet channels carries away the heat extracted from the primary air stream.
In the staged indirect evaporative cooling method, the primary air flows through the dry channels and the secondary air through the wet channels. As the primary air flows through the dry channels, small fractions of it are bled into the wet channel in multiple stages. The process of staged bleeding of the primary air into the secondary air stream flowing through the wet channels greatly increases the efficiency of the evaporative cooler. Whereas the conventional direct and indirect evaporative coolers can lower the dry bulb temperature of the primary air stream to within five to thirty percent (5% to 30%) of the wet bulb temperature of the air, the staged indirect evaporative cooler is capable of lowering the dry bulb temperature of the primary air stream up to twenty two percent (22%) below the wet bulb temperature and to within fifteen percent (15%) of the dew point temperature.
Direct and indirect evaporative cooling methods can be combined into compound evaporative cooling method. Another type of evaporative cooling method is the desiccant-assisted cooling method wherein the air cooled by evaporation is dehumidified by means of a regenerative desiccant material in order to increase its comfort cooling capacity, as described in the U.S. Pat. No. 4,002,040 to Munters et al. In this method, regeneration by heating the desiccant material is necessary to drive off the water absorbed by the desiccant material during dehumidification. The evaporative cooling method can also be used in conjunction with other methods of cooling, such as the vapor compression cooling. It is such an evaporation-assisted vapor compression cooling in a motor vehicle to which the subject invention pertains.